Malignant transformation by Rous sarcoma virus is induced by a virus-coded protein which plays no role in virus reproduction. Immunoselection techniques have identified a series of virus-specific proteins in a variety of avian and mammalian cells transformed by two strains of Rous sarcoma virus. Some of these proteins are viral structural proteins, or precursors of virion proteins. Several other unidentified proteins are candidates for the transforming protein, and more sensitive techniques are being applied to distinguish virion proteins from the transforming protein and its possible precursors. The function of the transforming protein has been examined using a virus mutant inducing temperature-dependent transformation. The protein apparently has an enzymatic modifying function, probably an involvement in glycosylation of proteins or lipids. This modifying action has a direct effect on cellular membranes resulting in an increased uptake of sodium ions. The unequivocal identification of a virus-coded transforming protein and resolution of its function appears attainable. BIBLIOGRAPHIC REFERENCES: Bader, J.P., Lew, M.A. and Brown, N.R.: Uptake of exogenous metabolites by virus-transformed cells: Changes induced by temperature and pH. Arch. Biochem. Biophys. 175: 196-208, 1976. Bader, J.P.: Sodium: A regulator of glucose uptake in virus-transformed and nontransformed cells. J. Cell. Physiol. 89: 677-682, 1976.